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Evolution of strontium isotope ratio with time for Phanerozoic (after Veizer et al., 1999). Inset shows the strontium isotope evolution of Earth's mantle and Oceans (after Faure, 1986).
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Chemostratigraphy or chemical stratigraphy deals with the correlation of sedimentary strata based on systematic variation of a particular chemical composition with time in the history of the Earth. In particular, this method is widely used in decoding the temporal variations seen in marine sediments that are deposited uninterruptedly in deep sea or...
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... has four naturally occurring stable isotopes ( 84 Sr, 86 Sr, 87 Sr and 88 Sr), of which 87 Sr is the daughter nuclide of rubidium ( 87 Rb). The concentration of strontium in naturally occurring rocks vary widely from less than one parts per million in ultramafic rocks (rocks that are common in Earth's mantle and consists almost entirely of ferromagnesian minerals with less than 45% SiO 2 contents) to more than few thousands of parts per million in marine carbonate sedimentary rocks. Seawater acts as the largest reservoir of strontium, by accumulating it from surface runoff throughout the history of the Earth. Strontium in the seawater has a long residence time (∼2.4 × 10 6 yr; Jones and Jenkyns, 2001) relative to a short mixing time (∼10 5 yr; Kump, 1991;Jacobsen and Kaufman, 1999), and therefore, global ocean is considered to be homogenous with respect to its 87 Sr/ 86 Sr ratio ( Halverson et al., 2007). The 87 Sr/ 86 Sr ratio of seawater is controlled by the mixing of continental derived strontium, which is radiogenic due to high rubidium content in granitic continental rocks and those from the volcanic sources, especially from the mid-ocean ridge basalts derived from the partial melting (the process though which some minerals in a rock melt initially and accumulate to form a magma) of mantle and are representative of mantle evolution of Rb-Sr isotope system (DePaolo, 1980). Carbonate rocks also act as sources for strontium, where it is recycled. In fact, carbonate minerals, precipitated directly from seawater, contain high concentration of Sr (few hundreds to thousands of ppm on average) and very low contents of Rb (parts per billion levels) resulting in low contribution of radiogenic Sr. Therefore, the 87 Sr/ 86 Sr ratio of marine carbonate sediments directly reflects contemporaneous sea water composition, irrespective of the age it has formed. This peculiar and systematic geochemical behavior of strontium with time in seawater and its record preserved in carbonate rocks is best suited for chemostratigraphic purpose. Previous studies, as early as in 1970's, have identified the application of strontium isotope evolution in seawater as a potential chronometer for Phanerozoic strata (Faure, 1977). High resolution biostratigraphy and precise age determination in representative geological sections have led to the refinement of the strontium isotope reference curve for the whole Phanerozoic era (Veizer et al., 1999, Figure ...
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... One of the most common applications of carbonate geochemistry is chemostratigraphy, which helps us to correlate sequences as well as to determine the apparent depositional ages of sedimentary rocks using a variety of geochemical proxies (Trønnes and Sundvoll, 1995;Veizer et al., 1999;Melezhik et al., 2001;Halverson et al., 2010;Satish-Kumar, 2015). When compared with other elements, strontium in seawater has a long residence time (~2.4 ...
In this contribution we review the possibility of establishing the depositional age and tectonic settings of metamorphosed carbonate rocks from continental collision zones in the East African-Antarctic Orogen. The geochemical characteristics of regionally distributed meta-carbonate rocks from the Highland Complex (HC) in Sri Lanka are considered in detail and compared with similar occurrences in East Antarctica, India, Madagascar and Africa. The variations seen in the Highland Complex of Sri Lanka imply that carbonate deposition was younging from west to east, spanning apparent ages from Mesoproterozoic to Neoproterozoic. In the case of East Antarctica, such variations are within the Neoproterozoic, whereas in southern India, Madagascar and Mozambique they have a broader age range possibly from the Paleoproterozoic to Mesoproterozoic. There is also clear evidence that some carbonates were deposited in an open ocean surrounding volcanic islands in the Mesoproterozoic. Shale-normalized REE patterns have typical signatures of open ocean deposition in a passive continental margin with variable continental input in platforms nearby to island arcs. In comparison to Phanerozoic equivalents, the absence of a Ce anomaly is most significant, whereas other parameters such as (Pr/Yb)SN, (Pr/Tb)SN, and (Tb/Yb)SN were used to evaluate relative enrichments of the LREE, MREE and HREE fractions that are characteristic of ambient seawater. Pronounced La, and Y anomalies with minor Eu and Gd anomalies and correlations of REE parameters and anomalies with carbon and oxygen isotopes, ⁸⁷Sr/⁸⁶Sr initial ratios and εNd values are evaluated for meta-carbonate rocks in the Proterozoic collision zone. The εNd values and Sr initial ratios suggest that basins in the western Mozambique Ocean that separated the East Gondwana from West Gondwana received contributions from Archean continental crust and ambient seawater, whereas the eastern Mozambique Ocean had REE contributions from specific cratonic continents in passive margins or from continental/volcanic island arcs in active margins.
